This invention relates generally to an EMI suppressor filter and, more particularly, to a combination capacitive and inductive filter employing a ferroelectric/ferromagnetic composite.
Electromagnetic interference (EMI) and radio frequency interference (RFI), typically in the form of stray radio frequency noise, cross-talk between electrical devices, spark discharges, poor or intermittent contact between metal bonds and electrical components, and atmospheric interference, can be a significant problem in the operation and performance of many electrical circuits. This problem becomes increasingly more pronounced as electrical circuits become smaller in size and the electrical components are positioned closer together. Additionally, low level signals in connection with computer systems and the like require better EMI and RFI filtering because the switching electronics in these systems operate at higher voltages.
Series capacitors are typically employed in the art to filter low frequency signals and series inductors are employed to filter high frequency signals and vice versa for shunt elements. State of the art low pass EMI/RFI filters typically include installing shunt capacitors on an electronic circuit board using conventional manufacturing technology. For certain applications, a series inductor is added to provide low pass xe2x80x9cLCxe2x80x9d filtering, such as when a block inductor is placed in series or parallel with one or more discrete shunt capacitors. The use of refined LC filtering is often necessary because capacitors may exhibit inductance at high frequencies, which can significantly reduce the effectiveness of an electronic device. As electronic devices become more compact, these types of filters take up increasingly valuable space on the circuit board. Furthermore, these filters do not always provide a sufficient level of protection because their passband is a function of frequency, and thus application dependent. As a result, it is often necessary to narrowly tailor the capabilities of such filters to perform well for specific applications.
It is also known in the art to position EMI/RFI filters, such as feed-through filters, at electrical interconnects to suppress cross-talk and other extraneous noise at connector pins. Conventional filters of this type can include a ferroelectric ceramic tube that is plated on its interior and exterior surfaces with a metallic coating that serves as a pair of electrodes. The interior electrode is in electrical contact with a connector pin and the exterior electrode is in electrical contact with ground. The capacitance of the filter depends upon the surface area and thickness of the tube and the dielectric constant, or permittivity, of the ceramic material used. These filter components are adequate for many applications.
It is known in the art to form a ceramic tube out of a ferromagnetic material, such as ferrite, and then sinter a ferroelectric material, such a barium titanate, to the exterior surface of the tube. The ferromagnetic material, characterized by having a high permeability, provide inductance and the ferroelectric material, characterized by having a high permittivity, provides, with the appropriate metallization, capacitance between the ferromagnetic material and ground. As a result, the ferromagnetic and ferroelectric materials act together to provide an LC filter, where the inductive and dissipative capability provided by the ferromagnetic material attenuates the interference which otherwise occurs with the capacitive element at the higher frequencies.
Although the known EMI/RFI filters made in this manner have advantageous features in terms of electromagnetic interference attenuation, multi-component filters are less simple to assemble and are believed to be less expensive to manufacture and store than single element filters.
To simultaneously provide both the capacitive and inductive filtering for EMI/RFI suppression, a number of desirable properties and characteristics for such a filter can be identified. Particularly, a material having a high DC resistivity would prevent shorting between adjacent connector pins; a material having a high dielectric constant would provide improved capacitance; a material having a high permeability would produce inductive capabilities; and a material having a significant mechanical strength would provide for durability in assembling. Some of these properties may be provided by one material and the remaining properties may be provided by another material. However, simply mixing two materials together will not produce a composite that will achieve the desired properties because of high porosity. If the mixture is sintered to remove the porosity, the permittivity/permeability is degraded becoming relatively low. This is because when the two materials are sintered at high temperatures to achieve the desired characteristics referred to above, the materials chemically react with each other resulting in lower permittivity/permeability and resistivity.
Materials are known in the art which exhibit both ferroelectric and ferromagnetic properties. One class of such materials consists of compounds having a single crystalline phase. However, the permeability and permittivity of this group of materials are generally inadequate for technical applications because the optimum magneto-electric properties of these components exist only at temperatures well below room temperature.
A more recently discovered group of magnetoelectric materials are formed from composites of fine grain powders of ferrite and lead zirconate titanate (PZT) which have been sintered together for evaluating magnetostrictive and piezoelectric effects, i.e., the contraction or expansion of a material when subjected to a magnetic or electrical field. However, lead is reactive with the ferrite, yielding a composite having greatly diminished permeability, permittivity, and resistivity as compared to its individual constituent materials. Such losses in constituent properties are well known to those skilled in the art.
A ferroelectric-ferromagnetic composite for use in an EMI/RFI suppression filter has been developed in the art that significantly solves the problems referred to above. Particularly, U.S. Pat. No. 5,497,129 issued Mar. 5, 1996, U.S. Pat. No. 5,512,196 issued Apr. 30, 1996, U.S. Pat. No. 5,601,748 issued Feb. 11, 1997 and U.S. Pat. No. 5,856,770 issued Jan. 5, 1999, all disclose ferroelectric-ferromagnetic composite materials for an EMI/RFI filter. Each of these patents is assigned to the assignee of this application, and are herein incorporated by reference. These patents disclose a ferroelectric-ferromagnetic composite that includes a ferroelectric material and a ferromagnetic material which are combined to form a solid composite material which is capable of suppressing electromagnetic interference in an electrical component or device. The grains of the ferroelectric material and the grains of the ferromagnetic material are combined, intermixed and consolidated to form the composite such that the ferroelectric and ferromagnetic grains substantially retain their respective, discrete electromagnetic properties.
In one embodiment, the ferroelectric material is barium titanate and the ferromagnetic material is a ferrite material, such as a copper zinc ferrite. The solid composite material is combined in a manner that insures that the microstructure of the solid ferroelectric-ferromagnetic composite is characterized by grains which are large enough to maintain their respective ferroelectric or ferromagnetic properties. Detrimental interaction between the ferroelectric and ferromagnetic materials is substantially absent so as to permit the materials to retain their permittivity and permeability properties for the desirable interference suppression.
The ferroelectric material selected for the ferroelectric-ferromagnetic composite is barium titanate (BaTiO3), although other suitable ferroelectric materials could be used, such as barium strontium niobate, and barium copper tantalate. Barium titanate is the preferred material in part because it is a high dielectric material having a large permittivity of about 1000 or higher at about 1 kHz. Further, the permittivity of barium titanate can be enhanced by the addition of dopants.
The ferroelectric material may have a sintering point ranging from about 1300xc2x0 C. to about 1400xc2x0 C. The ferroelectric material is chosen to have a sintering temperature above that of the ferromagnetic material, preferably at least about a 250xc2x0 C. higher sintering point than the ferromagnetic material, so that the ferromagnetic material diffuses around the ferroelectric phase. This allows the advantage of forming a structure of low porosity to provide a material having a higher permeability, permittivity and low dielectric loss. Both the ferroelectric and ferromagnetic materials are evenly distributed through the composite, preferably so that the sintered composite does not contain open pore porosity. This provides the advantage of low dielectric loss. In one embodiment, the ferroelectric-ferromagnetic composite has a closed pore porosity ranging from about 0% to about 10% by volume.
The ferromagnetic material selected for the ferroelectric-ferromagnetic composite is a ferrite, which is a high resistance magnetic material consisting principally of ferric oxide (Fe2O3) and one or more other oxides. The ferromagnetic material may have an AB2O4 formula, where component A is selected from the group consisting of Cu, Mg, Zn, Ni and Mn, and component B includes primarily Fe. Component A may also be selected to include a low sintering component that lowers the overall sintering temperature of the ferromagnetic material to about 250xc2x0 C. less than the ferroelectric component. Copper is the preferred low sintering component. Component A may also be selected to include a high electrical resistivity component such as Mg, so that the electrical resistivity of the composite is at least 106, 107, 109 or 1012 ohm cm. Mg may also be added as component A to insure high electrical resistivity. Component A may also be chosen to provide a high permeability component such as Zn, so that the permeability is at least 30 at 100 kHz or at least 1 at 100 MHz. The material may also be chosen to provide a high permeability, for example, 100 at 100 kHz. The ferrite is a copper-based ferrite because of the low sintering temperatures associated with such ferrites. Copper zinc magnesium ferrite with excess MgO (Cu0.2Mg0.4Zn0.5Fe2O4) is an example of such a copper-based ferrite. In a preferred embodiment, the composite provides electromagnetic interference suppression up to at least 1 GHz.
The ferroelectric-ferromagnetic composite materials disclosed in the patents referred to above have been successful in providing optimal properties for both capacitive and inductive filtering for EMI/RFI suppression. However, these patents do not discuss specific metallization and electrode configurations for a surface mounted EMI/RFI filter that incorporates both capacitive elements and inductive elements for the same dielectric made from the composite. It is an object of the present invention to provide a combination of both capacitive and inductive components in connection with a common ferroelectric-ferromagnetic composite for this purpose.
In accordance with the teachings of the present invention, an EMI filter is disclosed that makes use of a common ferroelectric-ferromagnetic composite in connection with capacitive and inductive elements. The geometrical structure of the filter is such that the electric fields and magnetic fields generated by the capacitive and inductive elements simultaneously penetrate the entire volume of the composite. The capacitive plates are aligned with the magnetic field, or are split to avoid setting up eddy currents in the plates which oppose the penetration of magnetic flux through the dielectric.
Various structural designs can be provided to define the capacitive elements and the inductive elements in different configurations relative to the composite to satisfy the desirable requirements. One particular design includes providing a slab of the composite having plates on opposite sides of the slab that define capacitors, and a conductive strip wrapped around the slab and electrically connected to the hot capacitive plates that define the inductor. This basic configuration can be extended to a cylinder made of the composite where a ground plane is provided on one surface of the cylinder, and the hot capacitor plates are provided on the other surface of the cylinder, where a conductive strip on the other surface defines the coil. A multilayer filter can be provided that includes layered substrates, where metallized vias extending through the substrates define a coil, and ground planes and capacitive plates on alternating substrates define the capacitive elements.
Additional objects, advantages and features of the present invention will become apparent from the following description and appended claims, taken in conjunction with the accompanying drawings.